Inheritance Part I

Lecture Objectives To learn about inheritance To understand how to inherit and override superclass methods To be able to invoke superclass constructors To learn about protected and package access control To understand the common superclass Object and to override its toString() and equals() methods

An Introduction to Inheritance Inheritance: extend classes by adding methods and fields Example: Savings account = bank account with interest class SavingsAccount extends BankAccount { new methods new instance fields }

An Introduction to Inheritance (Cont’d) The class SavingsAccoun t automatically inherits all methods and instance fields of the class BankAccount Extended class = superclass ( BankAccount ), extending class = subclass ( Savings ) Continued… SavingsAccount collegeFund = new SavingsAccount(10); // Savings account with 10% interest collegeFund.deposit(500); // OK to use BankAccount method with SavingsAccount object

An Introduction to Inheritance (Cont’d) Inheriting from class: subclass inherits behavior and state One advantage of inheritance is code reuse Every class extends the Object class either directly or indirectly Figure 1: An Inheritance Diagram

An Introduction to Inheritance (Cont’d) In subclass, specify added instance fields, added methods, and changed or overridden methods public class SavingsAccount extends BankAccount { private double interestRate; public SavingsAccount(double rate) { interestRate = rate; } public void addInterest() { double interest = getBalance() * interestRate / 100; deposit(interest); } }

An Introduction to Inheritance (Cont’d) Encapsulation: The method addInterest() calls getBalance() method rather than updating the balance field of the superclass (field is private ) Note that addInterest calls getBalance without specifying an implicit parameter (the calls apply to the same object)

Layout of a Subclass Object SavingsAccount object inherits the balance instance field from BankAccount, and gains one additional instance field: interestRate : Figure 2: Layout of a Subclass object

Syntax of Inheritance class SubclassName extends SuperclassName { methods instance fields }

Syntax of Inheritance (Cont’d) Example: public class SavingsAccount extends BankAccount { private double interestRate; public SavingsAccount(double rate) { interestRate = rate; } public void addInterest() { double interest = getBalance() * interestRate / 100; deposit(interest); } Purpose: To define a new class that inherits from an existing class, and define the methods and instance fields that are added in the new class.

Inheritance Hierarchies Sets of classes can form complex inheritance hierarchies Example: Figure 3: A Part of the Hierarchy of Ancient Reptiles

A Simpler Hierarchy: Hierarchy of Bank Accounts Consider a bank that offers its customers the following account types: 1.Checking account: no interest; small number of free transactions per month, additional transactions are charged a small fee 2.Savings account: earns interest that compounds monthly

A Simpler Hierarchy: Hierarchy of Bank Accounts (Cont’d) Figure 4: Inheritance Hierarchy for Bank Account Classes Inheritance hierarchy:

A Simpler Hierarchy: Hierarchy of Bank Accounts (Cont’d) All bank accounts support the getBalance() method All bank accounts support the deposit() and withdraw() methods, but the implementations differ Checking account needs a method deductFees() ; savings account needs a method addInterest()

A Simpler Hierarchy: Hierarchy of Bank Accounts (Cont’d) All bank accounts support the getBalance() method All bank accounts support the deposit() and withdraw() methods, but the implementations differ Checking account needs a method deductFees() ; savings account needs a method addInterest()

A Less Simpler Hierarchy: Hierarchy of Person Figure 5: Inheritance Hierarchy for Person-derived Classes

Inheriting Methods Override method:  Supply a different implementation of a method that exists in the superclass  Must have same signature (same name and same parameter types)  If method is applied to an object of the subclass type, the overriding method is executed Inherit method:  Don't supply a new implementation of a method that exists in superclass  Superclass method can be applied to the subclass objects

Inheriting Methods (Cont’d) Add method:  Supply a new method that doesn't exist in the superclass  New method can be applied only to subclass objects

Inheriting Instance Fields Can't override fields Inherit field: All fields from the superclass are automatically inherited Add field: Supply a new field that doesn't exist in the superclass

Inheriting Instance Fields (Cont’d) What if you define a new field with the same name as a superclass field?  Each object would have two instance fields of the same name  Fields can hold different values  Legal but extremely undesirable

Implementing the CheckingAccount Class Overrides the deposit() and withdraw() methods to increment the transaction count: public class CheckingAccount extends BankAccount { public void deposit(double amount) {...} public void withdraw(double amount) {...} public void deductFees() {...} // new method private int transactionCount; // new instance field }

Implementing the CheckingAccount Class (Cont’d) Each CheckingAccount object has two instance fields:  balance (inherited from BankAccount )  transactionCount (new to CheckingAccount )

Implementing the CheckingAccount Class You can apply four methods to CheckingAccount objects:  getBalance() (inherited from BankAccount )  deposit(double amount) (overrides BankAccount method)  withdraw(double amount) (overrides BankAccount method)  deductFees() (new to CheckingAccount)

Inherited Fields Are Private Consider deposit method of CheckingAccount Can't just add amount to balance balance is a private field of the superclass public void deposit(double amount) { transactionCount++; // now add amount to balance... }

Inherited Fields Are Private A subclass has no access to private fields of its superclass Subclass must use public accessors inherited from the superclass

Invoking a Super Class Method Can't just call deposit(amount) in deposit() method of CheckingAccount That is the same as this.deposit(amount) Calls the same method (infinite recursion) Instead, invoke superclass method super.deposit(amount)

Invoking a Super Class Method Now calls the deposit() method of BankAccount class Complete method: public void deposit(double amount) { transactionCount++; // Now add amount to balance super.deposit(amount); }

Syntax 13.2: Calling a Superclass Method super.methodName(parameters) Example: public void deposit(double amount) { transactionCount++; super.deposit(amount); } Purpose: To call a method of the superclass instead of the method of the current class